The present invention relates to integrated circuit fabrication and, more specifically, to photomask blanks, photomasks used in chip fabrication, and the manufacture of a photomask.
Phase shift masks are photomasks that leverage the interference generated by phase differences to thereby improve image resolution. The fabrication of a phase shift mask blank typically involves patterning a layer of a translucent material carried on a mask blank with an etching process to form a mask pattern. Light is projected through the mask pattern of the phase shift mask onto a wafer to expose a layer of a light sensitive material. It is important that during wafer printing, light passes only through the chip area of the mask. If light passes through the mask region outside of the chip area, that stray light distorts the chip pattern. Therefore, the phase shift mask blank includes a thick opaque frame at its periphery that interacts with the reticle masking blades (REMA blades) on a photolithographic stepper tool to block stray light. The thick frame must have a high optical density that is related to its thickness.
In conventional phase shift masks, the same opaque material layer used to form the frame is used as a hardmask to form the mask pattern by patterning the underlying layer of phase-shift material. This duality in function requires the opaque material layer to have a thickness (e.g., 50 nanometers or larger) that is far thicker than an optimal thickness for forming the mask pattern. For example, due to the thickness of the opaque material layer, a thick resist layer must be used to pattern both the frame and the mask pattern.